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By Jason Palmer (Image: Mary Caruso) Self-healing composite materials that can fix small cracks in the structures of planes, bridges, and wind turbines could become more cost-effective thanks to a new bonding mechanism discovered by researchers in the US. Engineers have high hopes for composite materials that can repair small cracks in their structure. “When you have any damage induced by fatigue, there’s usually nothing you can do except wait for catastrophic failure,” says Jeffrey Moore, who led the research at the University of Illinois at Urbana-Champaign, Urbana, US. Self-healing composites should change that. These materials contain capsules of a liquid adhesive which leaks out and repairs tiny cracks when they appear. (Watch a video showing how these materials might repair helicopter blades. Animation by the Beckman Institute Imaging Technology Group) However, the adhesives usually require some kind of post-processing to make them set, such as curing with UV light or heating to high temperatures. What engineers would prefer, though, is a material that healed itself without any extra intervention. In 2001, Moore’s group developed just such a material that relied on the mixing of two different chemicals that set like a two-part epoxy. The material contains two types of capsule: one containing a ring hydrocarbon called dicyclopentadiene and the other containing a ruthenium solvent that acts as a catalyst, causing the rings to break open and polymerise. Any crack causes the chemicals to mix and set, bonding the crack faces together. But ruthenium is rare. “An Airbus fuselage has 60,000 pounds of composites in it. If you used the catalyst approach, a significant fraction of the world supply of ruthenium would be flying around in one plane,” Moore told New Scientist. That makes it impractical for most applications, so his team set to work looking for an alternative. Seeking to improve the approach, the group changed to a nickel-based catalyst, but had to change the solvent as well. The first step was to gauge the new solvent in the absence of a catalyst. To their surprise, it worked almost as well. Moore says the solvent was probably dissolving the composite material, allowing it to mix and bond again, although he concedes the exact mechanism remains a mystery. The group then tested another solvent, using a chemical called chlorobenzene. After fracture and self-healing, the composites containing chlorobenzene recovered up to 100% of their original strength – as good as new. And, although toxic, chlorobenzene is a hundred times cheaper than ruthenium and is much more easily available. “It’s a very interesting way forward,” says Ian Bond, a self-healing materials researcher at the University of Bristol. “It really improves on the catalyst method.” However, Bond warns that the toxicity of chlorobenzene is likely to make the idea less industrially attractive. Moore’s team, meanwhile, is testing a number of less toxic, more biodegradable solvents to do the same job. Journal reference: Macromolecules (DOI: